1. Field of the Invention
The present invention relates to a thin film transistor. More specifically, the present invention relates to a thin film transistor and a method for fabricating the same.
2. Discussion of the Related Art
In accordance with the evolution of information-dependent society, demand for display devices has vastly increased. In response to this demand, in recent years, a variety of display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electro-luminescent displays (ELDs), and vacuum fluorescent displays (VFDs) are being researched and some thereof are utilized as displays for various equipment. Such a display device includes a thin film transistor formed as a switching device in each pixel region.
The thin film transistor includes an oxide thin film transistor (oxide TFT) which is a thin film transistor using oxide for a semiconductor layer, an organic thin film transistor (organic TFT) which uses an organic material for a semiconductor layer, an amorphous silicon thin film transistor (amorphous silicon TFT) which includes a thin film transistor substrate using amorphous silicon for a semiconductor layer, and a poly-silicon thin film transistor (polysilicon TFT) which includes a thin film transistor substrate using polycrystalline silicon for a semiconductor layer.
In particular, the oxide thin film transistor has advantages of higher charge mobility and lower leakage current characteristics, as compared to the silicon thin film transistor. Furthermore, since the silicon thin film transistor is formed by a high-temperature process and the oxide semiconductor layer is subjected to a crystallization process, the silicon thin film transistor is disadvantageous in terms of increase in area due to deteriorated uniformity during crystallization, as area increases. However, the oxide thin film transistor is suitable for a low-temperature process and is advantageous in terms of increase in usable area.
FIG. 1 is a sectional view illustrating a related art thin film transistor, which illustrates an oxide thin film transistor with a top gate structure.
As shown in FIG. 1, the general oxide thin film transistor includes a substrate 10, a light-shielding layer 11 formed on the substrate 10 and a buffer layer 12 covering the light-shielding layer 11. In this case, the light-shielding layer 11 prevents exterior light from being incident upon the semiconductor layer 13. Generally, the light-shielding layer 11 is made of a non-transparent metal such as molybdenum (Mo), chromium (Cr), copper (Cu), tantalum (Ta), or aluminum (Al).
In addition, the general oxide thin film transistor includes a semiconductor layer 13 formed on the buffer layer 12, and a gate insulating film 14a and a gate electrode 14 formed on the semiconductor layer 13 in this order. The interlayer insulating film 15 covering the gate electrode 14 is formed such that it exposes both edges of the semiconductor layer 13, and source and drain electrodes 16a and 16b are formed to connect to both edges of the exposed semiconductor layer 13.
In this regard, in the general oxide thin film transistor, the light-shielding layer 11 and the semiconductor layer 13 having different etching rates should be formed using different masks, so that the semiconductor layer 13 completely overlaps the light-shielding layer 11. Furthermore, generally, since the light-shielding layer 11 is formed of a non-transparent metal material and thus has a considerably low resistance, the buffer layer 12 should have a sufficient thickness.